Message box when user clicks on any of several fields on a form

Instead of displaying a message box, it may be a better approach to use a visual clue that the field is locked, such as changing the text color or the background color. By displaying the message box, an inadvertent click in one of the fields will cause the message box to display and what seems to be a benign alert can quickly become an large annoyance.

Also, if for some reason your user wanted to highlight text in the locked control (textbox) to copy/paste the text elsewhere, this message box again would be an annoyance.

I very much do appreciate your insight.  But in this case we really do want a message box to appear.  Can it be done without having to write the code behind each field's onclick event?